The following information is provided to assist the reader in understanding technologies disclosed below and the environment in which such technologies may typically be used. The terms used herein are not intended to be limited to any particular narrow interpretation unless clearly stated otherwise in this document. References set forth herein may facilitate understanding of the technologies or the background thereof. The disclosure of all references cited herein are incorporated by reference.
Congestive heart failure or CHF affects an estimated 5.7 million persons in the United States alone. Increasingly, heart assist devices are being implanted in a patient's body to assist the patient's weak heart, by increasing the blood flow to the body. Such increased blood flow alleviates the symptoms of congestive heart failure, and returns the patient to a normal or near normal state of health.
As used herein, the term “powered” generally refers to the use of electrical power. As used herein, the term “implanted” refers to a medical device either partially or completely inserted into the body of a patient, for example, a human patient. The term “thoracic aorta” or “TA” refers to the descending thoracic aorta. Commonly, the term ventricular assist device or VAD is used to describe pumps that help the heart deliver more blood to the body. For simplification, the term “VAD”, as used herein, describes any device or system that mechanically helps the heart pump more blood. VADs traditionally have used positive displacement collapsing pumping chambers having inlet and outlet valves to force forward blood flow in a pulsing manner. Such positive-displacement, pulsing VADs have been physically too large for use in small patients. State-of-the-art VADs have rotating impeller pumps that slice and push blood forward and are significantly smaller the older VADs to fit in any sized adult patients. However, state-of-the-art rotating impeller VADs have a number of serious drawbacks, including, blood damage, infection risk and the fact that they are not failsafe. The impeller blades of such VADs operate at high speeds (for example, 3,000 to 8,000 revolutions per minute) and impart high shear stress to the blood components including red blood cells, platelets and a high molecular weight protein called the von Willibrand factor. Also, in some cases, rotating VADs use the blood itself as a bearing material, which is a source of substantial shear. Because of shear related blood damage, patients with implanted rotary VADs often experience excessive bleeding and clotting, leading to brain damage, strokes and/or the need for blood transfusions. Additionally, because rotary VADs are placed in parallel with the heart's left ventricle, the loss of pump power may be fatal in an estimated 40 percent of the patients. This risk arises with loss of power because the non-rotating impeller pump, which is placed in parallel with the left ventricle, becomes a shunt path for high pressure arterial blood to flow backward into the weak low pressure left ventricle, overloading it into a severe state of failure.
A number of VADs have used the principle of counter pulsation to boost blood flow from the left ventricle to the body. For example, a hot-dog shaped intra-aortic balloon may be inserted into the thoracic aorta or TA through a minimally invasive femoral artery incision and pulsed with gas pressure and vacuum to alternately inflate and deflate the balloon. The balloon inflation is timed to occur early in diastole, pushing blood to the body during the heart's resting and filling time period, and to deflate late in diastole or early systole, making it easier for the heart to empty its blood into the aorta.
In other words, intra-aortic balloon counter pulsation works by adding energy and blood flow to the circulation during the balloon inflation time and by lowering the impedance against which the heart pumps during balloon deflation. Balloon deflation removes volume from the aorta just as the left ventricle begins pumping, and an incremental amount of blood is ejected from the left ventricle because of the reduced aortic impedance.
The pioneering cardiovascular surgeon Adrian Kantrowitz devised a permanently implanted counter pulsation system using an intra-aortic balloon equivalent, surgically attached to the descending thoracic aorta or TA. Kantrowitz described his device as an auxiliary mechanical ventricle or AMV. The Kantrowitz device includes a chamber having a flexible membrane that is sewn into the front wall of the TA. A gas conduit traverses the patient's skin to inflate and deflate the TA-appended chamber, producing a beneficial counter pulsation effect.
U.S. Pat. No. 7,347,811 describes a fluid chamber appended to the outside wall of the ascending aorta and having a flexible membrane that, when energized by high pressure fluid, invaginates a portion of the ascending aorta, effectively adding pumped volume to the aorta during diastole. Subsequently, the fluid is withdrawn during early systole to lower the impedance seen by the blood ejecting left ventricle. This counter pulsation device has volume changes in the 20 to 30 milliliter range.
Such counter pulsation devices use electric motors as a first mechanical energy source to pressurize an intermediate fluid that, in turn, pumps blood in the intra-aortic device or the appended fluid chamber. Such expansion drives blood in the aorta through the small vessels of the body.